In recent years, dielectric barrier discharge ionization detectors (which are hereinafter abbreviated as the “BIDs”) employing the ionization by dielectric barrier discharge plasma have been put to practical use as a new type of detector for GC (for example, see Patent Literatures 1 and 2 as well as Non Patent Literature 1).
BIDs described in the aforementioned documents are roughly composed of a discharging section and a charge-collecting section which is located below the discharging section. In the discharging section, a low-frequency AC high voltage is applied to plasma-generating electrodes circumferentially formed around a tube made of a dielectric material, such as quartz glass (“dielectric tube”), to ionize an inert gas supplied into the tube line of the dielectric tube and thereby form atmospheric-pressure non-equilibrium plasma. Due to the effects of the light emitted from this plasma (vacuum ultraviolet light), excited species and other elements, the sample components in a sample gas introduced into the charge-collecting section are ionized. The resulting ions are collected through a collecting electrode to generate detection signals corresponding to the amount of ions, i.e. the amount of sample components.
FIG. 7 shows the configuration of the discharging section and surrounding area in the aforementioned BID. As noted earlier, the discharging section 610 includes a cylindrical dielectric tube 611 made of a dielectric material, such as quartz, the inner space of which forms a passage of inert gas, such as helium (He) or argon (Ar) gas. On the outer wall surface of the cylindrical dielectric tube 611, three ring-shaped metallic electrodes (made of stainless steel, copper or the like) are circumferentially formed at predetermined intervals of space. A high AC excitation voltage power source 615 for generating a low-frequency high AC voltage is connected to the central electrode 612 among the three electrodes, while the electrodes 613 and 614 located above and below the central electrode are both grounded. Hereinafter, the central electrode is called the “high-voltage electrode” 612, while the upper and lower electrodes are called the “ground electrodes” 613 and 614. The three electrodes are collectively referred to as the plasma generation electrodes. Since the wall surface of the cylindrical dielectric tube 611 is present between the passage of the inert gas and the plasma generation electrodes 612, 613 and 614, the dielectric wall itself functions as a dielectric coating layer which covers the surface of those electrodes 612, 613 and 614, enabling dielectric barrier discharge to occur. With the inert gas flowing through the cylindrical dielectric tube 611, when the high AC excitation voltage power source 615 is energized, a low-frequency high AC voltage is applied between the high-voltage electrode 612 and each of the upper and lower ground electrodes 613 and 614 located above and below. Consequently, an electric discharge occurs within the area sandwiched between the two ground electrodes 613 and 614. This electric discharge is induced through the dielectric coating layer (the wall surface of the cylindrical dielectric tube 611), and therefore, is a form of dielectric barrier discharge, whereby the inert gas (plasma generation gas) flowing through the cylindrical dielectric tube 611 is ionized over a wide area, forming a cloud of plasma (atmospheric-pressure non-equilibrium plasma).
In the BID configured in the previously described manner, the dielectric layer which covers the surface of the plasma generation electrodes prevents an emission of thermions or secondary electrons from the surface of the metallic electrodes. Furthermore, since the plasma generated by the dielectric barrier discharge is a non-equilibrium plasma with low-temperature neutral gas, various factors which cause a fluctuation of the plasma are suppressed, such as a temperature fluctuation in the discharging section or an emission of gas from the inner wall of the quartz tube due to the heat. As a result, the BID can maintain plasma in a stable form and thereby achieve a higher level of signal-to-noise ratio than the flame ionization detector (FID), which is the most commonly used type of detector for GC.